This invention relates to a high precision liquid metering piston pump and externally mounted and actuated valves for accurate dispensing of liquids and that is capable of being cleaned and sterilized without disassembly, through the use of water, cleaning and sanitizing solutions, and steam. Such a liquid metering and dispensing system offers significant advantages in the automatic aseptic filling of pharmaceutical and other liquid products that must be filled into containers in an aseptic and contamination free environment.
Piston pumps for metering and dispensing liquid products are in common use, and offer a reliable and precise means of metering and delivering a wide variety of liquids. However, in most cases the designs are such that they cannot be cleaned, decontaminated and sterilized without first dismantling them into their constituent parts. Their subsequent assembly after cleaning and sterilization (where applicable), imposes a risk of recontamination of the devices from human and environmental factors.
The invention described herein, provides the benefits of precision, versatility and reliability of other piston pumps and their valves, while overcoming the disadvantages related to their need to be dismantled for cleaning, decontamination and sterilization. The invention also overcomes certain problems associated with piston pumps and valves that can be cleaned and sterilized without disassembly as they exist in prior art.
In the prior art designs of such types of devices, the piston is reciprocated and the cylinder is held stationary. Ingress and egress of the liquid product is through a port within the cylinder. In equipment that utilize these devices, the pumps are mounted vertically and the reciprocating actuators for the pistons are located beneath the pumps. This configuration allows the equipment to conform to the regulations relating to "Current Good Manufacturing Practices" of the Food and Drug Administration (FDA) and permits their use for metering and pumping liquids under clean and aseptic conditions, liquids such as parenteral drugs and certain food products. In such an arrangement however, the guide/seal area between the piston and cylinder is located beneath the cylinder cavity.
This presents a formidable challenge in both the design as well as the high tolerance fabrication of these pumps; particularly in the seal/guide region, where diametrically conflicting requirements need to be resolved. Since no elastomeric or other compliant seals can be used in this region (due to its inability to assure cleaning and sterilization in place), a tight clearance between the moving piston and stationary cylinder is required to minimize liquid product leakage. However too tight a clearance impedes hydrodynamic lubrication of the surfaces and can result in binding of the piston. This is especially exacerbated when pumping viscous liquids, suspensions or near saturated solutions.
This intractable problem is presently addressed by either tolerating a certain leakage loss of the liquid product through the seal/guide region or by using tighter clearances and separately and externally lubricating the piston with clean water. Neither of these solutions appear very desirable. In the former case, many liquid drugs are extremely expensive and often toxic. Their loss through leakage can be monetarily significant, as well as have the potential of exposing operators to toxic substances. The latter solution on the other hand can cause product contamination and cannot therefore be used with many kinds of products.
Among the valves used in such prior art designs, a popular type is the rotary spool valve usually mounted directly above the cylinder. These valves are also designed and fabricated to work without the use of elastomeric or other compliant seals. Actuation of the valve during pump operation is rotary. However, to set up for cleaning and sterilization, the valve spool is shifted longitudinally (along its axis) to expose the liquid contact areas, particularly in the seal regions. Separate actuators are required to shift this spool, adding to the cost and complexity of the system. In addition, the rotary sliding motion of the valves causes wear and sheds particles which enter the product stream.
Another type of valve used with these pumps is one in which a flexible diaphragm is used to isolate the liquid product contact area from the valve actuation device. In these designs, the flexible diaphragm has a limited life which is shortened with exposure to pressurized steam. This is particularly true with elastomeric diaphragms. Less so with metal diaphragms. As a result, such valves are subject to sudden and unpredictable failures. Worse, undetected pin holes develop in the diaphragm which can lead to loss of sterility of the product as well as its contamination.
Various liquid metering systems are presently available that are used for dispensing sterile pharmaceutical liquids and that can be cleaned without disassembly. Cleaning the metering systems without disassembly offers some advantages in the maintaining of sterility of the liquid drugs dispensed, in that a potential contamination hazard due to disassembly, cleaning and sterilization and subsequent reassembly of the pump systems (liquid product contact parts) is eliminated.
Of the various liquid metering and dispensing systems available for this purpose, those utilizing piston pumps and mechanical or electro-mechanically operated valves, tend to be the most accurate and reliable. There do exist piston pump and valve systems capable of being cleaned and sterilized without disassembly, in prior art; notably U.S. Pat. Nos. 4,688,611, and No. 4,832,092 assigned to Shibuya Kogyo. Also, U.S. Pat. No. 4,638,925 assigned to Robert Bosch GMBH.